Edit: I wrote too much, so I will boil it down: To me, the union between a man and woman is "special" bc of the potential for each man and woman to be born able to reproduce. Homosexuals are never born with the potential to reproduce amongst themselves. Not every male and female reach that potential, but the potential is their natural, default state.
Homosexual unions are not suffering from an abnormality that prevents them from reproducing with each other. They never could by default or nature, and never will naturally reproduce..... maybe someday with a lot of technological advancement they could be able to, but that's a different story.
Then, again, you're talking about spirituality.
Because, biologically, there's no distincation in "capability to reproduce" between a woman with a historectomy and a gay man. Neither is capable of bareing children without massive scientific assistance.
You can even take a woman who has a genetic defect that makes her unable to have children, such as serious autoimmune disesease and/or endometriosis. Someone who never had the potential to become pregnant.
I think maybe what you mean is that evolution, or whatever process created man-kind, has an evolved attachment that is different between a man and a woman than a man and a man. But I would argue that homosexual behavior is just as natural to a gay man or woman as hetersexual behavior is to a gay man or gay woman. Like, by definition, it is when that special attachment is aimed at the same gender.
I don't know... I mean, I can't really say I know you're wrong because I don't know any gay couples that have actually made lifetime commitments to eachother. But I can say I have met plenty of straight couples whos relationships were anything but "special."
And it's hard to have this discussion without including some views on morality (religious, utilitarian, whatever). From a position of rational moral law, two people should be allowed to enter into any kind of living contract they wish and there should be no LEGAL distinction in genders or in number of people entering into the contract.